Conventionally polyimides are prepared by curing polyamic acid to remove a water molecule and convert the polyamic acid to a polyimide structure. Typically this curing is done at an elevated temperature in various types of ovens. In the case of films, the films are heated to cure the polyamic acid to polyimide throughout the entire body of the film and the film is then spooled or otherwise supported. With this type of bulk curing, i.e. curing over the entire body of the article, in many cases the curing process is incomplete and also the curing process will provide a mixture of end products which can include isoimides and transimides as well as polyimides. The polyimides and isomides are essentially linear and not branched whereas the transimides are a branched structure. These reactions of curing polyamic acid (PAA) to the polyimide, isoimide and transimide structures are as follows. ##STR1##
It has been found that the non-linear nature of the transimide structure results in a lower surface density as compared to the linear structure of the polyimide which provides a much higher density surface. This presence of the lower density structure of transimide in any appreciable amounts will result in an undesirable surface condition which allows the penetration into the surface of moisture which can migrate therein via ionic solvents. The presence of the moisture will tend to swell and plasticize the polyimide film which in turn degrades the surface properties and, in particular, may lead to tensile stresses at inner facial regions with embrittlement, delamination and stress cracking being prevalent. This can and does significantly lower the strength of a bond between any metalization or metal layer applied to the surface of the polyimide and the underlying substrate. Thus, in cases where metal is to be applied to the surface of a polyimide substrate, it is desirable to reduce to a minimum and preferably eliminate any low density branched structures such as transimides.
While the prior art has recognized the desirability to increase certain surface properties of polyimides, nevertheless, none have suggested a process for improving the surface properties for the reception of metal or metalization on the surface thereof. For example, U.S. Pat. No. 3,361,589 teaches a treatment of the surface of a polyimide with a base for the purpose of improving the adhesion of the polyimide to organic adhesives which may be applied thereto. This process is also taught in U.S. Pat. No. 3,505,168. U.S. Pat. No. 4,440,643 teaches a process of applying a coating of polyamic acid onto the surface of an existing polyimide and curing the added polyamic acid to provide an essentially dual layer polyimide structure. The following U.S. patents suggest various polyimide materials and various types of surface treatments for various purposes; 3,179,633, 3,416,994, 4,623,563, 4,731,287, 4,543,295, 3,770,573, 3,770,528, re. 29,134 also the following IBM Technical Disclosure Bulletins suggest various curing processes for polyimides.
Volume 28, No. 6; dated November 1985 pages 2646 and 2647; PA1 Volume 24, No. 10; dated March, 1982, page 5110.
However, none of these patents or articles teach or suggest a method of improving the surface of an existing polyimide for the reception of a metal layer thereon.